The launch starts the third mission in three years for the robotic X-37 fleet, assembled in Boeing's now-shuttered Building 31 in Huntington Beach, California, for an estimated $1 billion apiece. But for all the time spent in orbit by the two school bus-size spacecraft – 693 days in all – it's no more clear today precisely what the Air Force has been up to with the X-37s.

Officially, the solar- and battery-powered X-37s are strictly experimental craft, meant to "demonstrate technologies for a reliable, reusable, unmanned space test platform" while also "operating experiments which can be returned to, and examined, on Earth," according to the Air Force.

“Take a payload up, spend up to 270 days on orbit," is how Gary Payton, Deputy Undersecretary of the Air Force for Space Programs, explained the X-37's mission. "They’ll run experiments to see if the new technology works, then bring it all back home and inspect it to see what was really going on in space.”

But the Air Force has consistently declined to state exactly what those experiments might be. And in theory the X-37s could also carry spy sensors or gear for hacking enemy satellites. That and the lack of specific information has elicited protests from the Chinese government, which has vowed to build a space plane of its own.

The opacity also makes a mockery of the State Department's international campaign for greater space transparency.Just last week in Vietnam, Deputy Assistant Secretary of State for Arms Control, Verification and Compliance Frank Rose quoted former U.N. head Boutros Boutros-Ghali. "To avoid conflicts based on misperceptions and mistrust, it is imperative that we promote transparency and other confidence-building measures – in armaments, in threatening technologies, in space and elsewhere,” Rose said.

Some observers question the space plane's official story. "Because it is an Air Force project and details about it are classified, and because it does not have a clear mission compared to simpler systems, this project has generated confusion, speculation and in some cases concern about its purpose," the Union of Concerned Scientists, a Massachusetts-based think tank, stated in a November fact sheet.

The concern could be warranted. The X-37B is a scaled-up version of NASA's X-37A, abandoned by the space agency in 2004. Able to deliver small payloads from its pickup truck bed-size bay, nimbly maneuver across orbits and automatically land like an airplane once its fuel reserves are depleted, the X-37B is essentially a miniature edition of NASA's Space Shuttle, retired 18 months ago after three decades of service.

But unlike the 120-ton Space Shuttle, the six-ton X-37 is too small to deploy large satellites or support complex space experiments, the kind that might require a human being present.

With no human crew, the X-37B's design doesn't really make sense for scientific use, the UCS points out. "The ability to return to Earth carries a high cost," according to the think tank's fact sheet. "Many missions in space do not require bringing a spacecraft back to Earth, and the space plane makes no sense for those. And even in cases when return does make sense, a spacecraft can land using a parachute rather than wings and landing gear."

"While this 'space plane' could perform a range of missions, in each case we can identify a better, more efficient, and/or cheaper way of doing each of those tasks," UCS concluded.

In the absence of a clearer explanation from the Air Force, John Pike from the Virginia-based Globalsecurity.org claimed, half-jokingly, that confusion was the whole point. "It acquired a life of its own," Pike said of the X-37. "And now to the extent that it might be said to have any larger purpose, it would be to bewilder the Chinese.”

Now, we're sure that's not true. But that doesn't mean we have any idea what the space planes are really for.